If you have to pick 4 subjects and you love maths, then pick further maths. You may be worried about the difficulty of it, but it shouldn't be any more difficult than chemistry- that's for sure. Before A-levels, I thought that I would breeze through the sciences and find maths really tricky but in fact, I have found the opposite. That's just my personal insight though.
Have you considered any other options such as psychology (could be useful for medicine?), a language or even a creative subject such as art? There's no shame in doing a 'soft' creative subject, especially if you're really good at it. I wish I had known that in Year 11!
Further maths sounds really interesting though, I wish I had chosen it! Doing A-level maths has made me love maths so much more than I did in Year 11.